Electroluminescent color image reproduction



May 14, 1957 B. KAZAN 2,792,447

ELECTROLUMINEZSCENT COLOR IMAGE REPRODUCTION Filed April 21, 1955 2Sheets-Sheet 1 1/ Z2 /6 .mwz

1 m cr/zw J2 May 14, 1957 B. KAZAN 2,792,447

ELECTROLUMINESCENT COLOR IMAGE REPRODUCTION Filed April 21, 1955 2Sheets-Sheet 2 IN V EN TOR.

ELECTROLUMINESCENT COLOR IMAGE REPRODUCTION Benjamin Kazan, Princeton,N. J., assignor to Radio Corporation of America, a corporation ofBeiaware Application April 21, 1955, Serial No. 502,963

4 Claims. (Cl. 178-54) devices have generally required the use of aplurality of projection tubes each adapted to provide an illuminatingbeam of radiation which may be a beam of visible light of a particularcolor. This required the use of illumination sources each providing amonochromatic beam through the use of optical filters or a selectedphosphor source. Each of these systems is, of course,

basically a system for converting electrical energy into radiant energyto provide a composite visual image in color.

One means for converting electrical energy into light energy utilizesthe principle of electroluminescence, wherein a phosphor is excited bythe application of a voltage or an electric field to the phosphor.Particles of a suitable phosphor may be embedded in a plastic and anelectric field applied to the phosphor by conducting sheets placed inclose association with the plastic.

An object of this invention is to provide an improved means forreproducing light images in natural or true color which utilizes theprinciple of electroluminescence.

Another object of this invention is to provide light image in full colorof relatively large area from a modulated electrical signal whileutilizing light sources each of which may provide ilumination of thesame or uniform color composition and without the use of color filters.

In accordance with this invention, light images in natural color may bereproduced by'varying the electrical field across elemental portions ofa layer of electroluminescent material in accordance with a plurality ofelectrical signals by means of a photoconductive layer placed in closeassociation with an electroluminescent layer. A plurality of individualbeams of light or radiant energy, which are individually and separatelymodulated with the signal representative of the component color of animage, are developed in such a manner that only a beam representative ofa given color component may irradiate those portions of thephotoconductive layer associated with an electroluminescent materialcapable of producing the given color.

The novel features that are considered characteristic of this inventionare set forth with particularity in the appended claims. 'The inventionitself, however, 'both as to its organization and method of operation,as well as additional objects and advantages thereof, will best beunderstood from the following description when read in connection withthe accompanying drawings, in which: Figure 1 illustrates schematicallyone embodiment of the present invention for reproducing a light image incolor wherein a plurality of flying spot scanners, each being amplitudeor intensity modulated in accordance with signal information, areutilized to energize an elec- United States Patent 2,792,447 PatentedMay 14, 1 957 troluminescent device under the influence of a foraminousmember or aperture mask;

Figure 2 shows a perspective view of a portion of the aperture mask andelectroluminescent device of Figure 1 illustrating the relativepositioning of the several elements;

Figure 3 is a side elevation view of an electroluminescent device andlens system or array provided in accordance with the present invention;

Figure 4 is a perspective view of a further arrangement for reproducinga televised image in natural color in accordance with the presentinvention; and

Figure 5 is a schematic showing of an electroluminescent target and beaminterceptor of opaque material having a plurality of openings orapertures therein.

Referring now to the drawings and particularly to Figure 1, there isshown electroluminescent device 10 adapted, when properly energized, toreproduce a color image. One form of the electroluminescent devicesuitable for use in the practice of this invention comprises a sandwichtype or" construction including a luminescent layer 11 and aphotoconductive layer 12 separated by and contiguous with an opaque orsemiopaque layer 13. More specifically, the sandwich includes on oneside a transparent base member 15 which may be glass, having depositedthereon a transparent conductive layer or coating 16. On the other side,there is employed a similar glass base member 17 also coated on itsinside surface with a transparent conductive layer or coating 18. Theluminescent layer 11 and photoconductive layer 12 are sandwiched betweenthe base members 15 and 17 and are in-contact with each other and theconductive layers 16 and 18.

In .order to enable the reproduction of a color image, theelectroluminescent layer 11 is divided, in one way or another intoelemental regions each of which is smaller than a picture element. Theindividual regions are arranged in a desired sequence and each producesa particular color of electroluminescent light. The desired elementalregions may be a parallel line arrangement, as illustrated in Figure 2,and may be obtained by dividing the electroluminescent phosphors intoelemental regions each having an appropriate color response.

For example, the repeating series of elemental regions R, B and G, shownin Figure 2 for a three color system, may be silver activated zincsulfide for blue, manganese activated alpha wellemite for red, andchromium activated aluminum berylliate for green.

In preparing the luminescent layer or sheet, the particles of phosphormaterial are mixed with or embedded in a light transmitting insulatingmaterial, i. e. a plastic, lacquer, wax or the like.

According to one method of preparing the luminescent body, a unitquantity of a plastic matrix material for the phosphor was prepared withthe following ingredients in approximately the given quantities:

This mixture was blended with approximately two grams of finely dividedphosphor particles, for example, copper activated zinc sulfide particleshaving a diameter of the order of one to five microns. The mixture ofplastic and phosphor was then ball milled for approximately one hour.Finally, the milled preparation was sprayed into "a suitable base plate.In an electro- 3 luminescent panel actually built, the thickness of thislayer was of the order of 1 to 3 mils.

The photoconductive layer 12 may be made of any photoconductive materialsensitive to the type of radiation to be employed in activating thelayer, and may be made in a manner similar to that described above forbeing adapted to make low resistance contact to one another in thepresence of incident radiation in the range between 4000 A. and 9000 A.A photoconductive body may then comprise a mass of this photoconductingpowder with or without a binder.

One method for producing a photoconducting powder comprisesreciystallizing a material selected from the group consisting ofsulphides, selenides and sulpho-selenides of cadmium to a desired rangeof particle sizes, incorporating into the recrystallized materialactivator proportions of a halide and activator proportions of a metalselected from the group consisting of copper and silver. By carefullycontrolling the firing process, the surfaces of the particles of saidrecrystallized material make low resistance electrical contact to oneanother in the presence of incident radiation in the range between 4000A. and 9000 A. By providing such low resistance electrical contactbetween particles, the photosensitivity of the particles is unmasked,facilitating the flow of photocurrents through a body of the powder.

The photoconductive layer may have a thickness comparable to that of theluminescent layer. The relative thickness of these layers is determinedby the types of materials involved and the desired voltage drop acrosseach layer when the electroluminescent device is in the unenergizedcondition.

The conductive layers 16 and 18 may one or both, be constituted of metalplates, grids, meshes or sheets or films of material adapted to betransparent to the type of radiation to be employed in operation of theelectroluminescent device. One method of forming these layers is toapply transparent conductive'material to the base plates 15, 17 havingthe desired radiation transmitting qualities. The transparent conductivematerial may be of the type formed by deposition of the vapors ofstannic chloride, water and methanol.

An additional opaque or semiopaque layer 13 may be interposed betweenthe photoconductive layer 12 and the electroluminescent layer 11 tolimit the amount of'light feedback to the photoconductive layer 12 fromthe electroluminescent layer 11.- Such a layer may be of a density toallow enough light to pass to utilize regenerative action. However,where the device is to be used for reproducing varying light images,this layer should be sufliciently opaque to preclude any possibility ofenough light being fed back to result in self energization. Inoperation, the elemental areas of the photoconductive layer 12 becomeconductive in accordance with the intensity. of the incident radiationon each of the elemental areas. As a result of the increasedconductivity of the photoconductive elements, the correspondingelemental areas of the electroluminescent layer have voltage incrementsapplied across them. The increased voltage across the electroluminescentareas causes corresponding increases in output light. Since theelectroluminescent device is comparatively thin, light emitted from agiven area of the electroluminescent layer represents radiation strikingthe corresponding area of the photoconductive layer. I

It is, of course, to be understood that the particular 4electroluminescent device illustrated is but one form applicable to thesystem of the present invention. Other devices having a layer ofphosphor material comprising elemental areas subdivided in a particularmanner for increased efiiciency or sensitivity may be utilized. In adevice having an elementally divided phosphor material it is requiredthat the color elements of the electroluminescent material be accuratelyregistered with the elements of photoconductive material in order toselectively energize the subelemental color areas.

A plurality of flying spot scanners, which may be, for example,cathode-ray tubes 20, 21 and 22, are used to energize selected areas inaccordance with the color information communicated by each tube. Threetubes are shown to illustrate the use of the present arrangement in athree color system; however, two or more tubes may be utilized inaccordance with the present invention with an appropriate luminescentdevice depending on the color system employed for signal transmission.Suitable operating potentials may be applied to each of the cathoderaytubes 20, 21 and 22.

Signal information necessary to modulate the beam intensity of each ofthe tubes 20, 21 and 22 may be supplied from a signal source 27 such asa conventional color television signal receiver or signal generatoradapted to derive signal information representing the individual primarycolor information concerning a televised object. The signal source 27,may supply synchronizing pulses to a deflection apparatus 28, which inturn supplies the yokes 29, 30 and 31 with deflection currents forpurposes of deflecting the electron beams in the cathode-ray tubes 20,21 and 22 in synchronism with each other and in synchronism With thetransmitted information.

Interposed between the electroluminescent device 10 and the cathode-raytubes 20, 21 and 22 is a foraminous beam intercepting structure or mask32 which is so arranged that the irradiating beam from any onecathoderay tube may reach only definite lines or points of theelectroluminescent device 10 which will reproduce the particular colorin accordance with the signal representations with which the particularbeam is modulated.

The beam or flying spot produced by each of the cathode-ray tubes may beoptically focused on the apertures of the slotted or foraminous mask 32by means of individual lens systems shown diagrammatically at 33, 34 and35.

Voltage from source 37 is applied across the electroluminescent device10 by means of the conducting layers previously described.

In operation, the embodiment of Figure 1 provides an output image on theside of the electroluminescent device 10 opposite from thephotoconductive layer 12 on which the flying spots are focused. Theflying spots energize elemental areas of the photoconductive layer 12which reduces the impedance across elemental areas of thephotoconductive layer 12 so that an increased voltage is applied acrosselemental areas of the electroluminescent layer 11. Since the incidentradiant energy is modulated in accordance with signal information, andsince the variation of the impedance of the elemental areas of thephotoconductive layer 12 varies with a variation in illumination, theamount of electroluminescent light emitted from each elemental area isdetermined by the modulation of the cathode-ray tubes 20, 21 and 22. Inthis manner,

a color image is elementally reproduced by the electroluminescent device10.

In general, electroluminescent layers energized by an alternatingvoltage produce two maxima of light during each A.-C. cycle rather thancontinuous light. As an unmodulated flying spot from a cathode-ray tubescans the device with an A.-C. voltage applied to the device. theelectroluminescent light will vary periodically at twice the A.-C.frequency. As a result, if signals are .used for intensity modulatingthe incident radiation and assuming a photoconductive material having avery fast response characteristic, the A.-C. frequency must be at leasthalf as high as the highest signal frequency employed. Also, the A.-C.frequency must be sufliciently high relative to the speed of the flyingspot so that the electroluminescent light will not appear in the form ofa sequence of dots or dashes.

However, photoconductive materials presently known generally have adecay time greater than one microsecond. it is therefore not requiredthat the A.-C. signal voltage be synchronized with the scanninginformation. Synchronization would be required if the decay time of thephotoconductive phosphor were less than the time of one cycle of theA.-C. voltage in order to insure emission of electroluminescent light atthe moment of excitation of the photoconductor.

The details of the mask 32 and the electroluminescent device It may bemore readily seen from an examination of Figure 2. The mask 32 comprisesaplurality of opaque members 40 which are separated by a plurality ofapertures in the form of line openings or slits which are positionedwith respect to the regularly recurring subelemental portions of theelectroluminescent layer 11 so as to allow the illuminating beam from asingle one of the plurality of beam sources or cathode-ray tubes toapproach the device from a particular direction and therefore toilluminate a particular one of the red, green or blue strips. It is tobe noted, however, that the illumination provided by any one of thebeams does not impinge directly upon the corresponding strip in theelectroluminescent layer, but, instead is caused to impinge upon theassociated portion of the photoconductive layer.

As above discussed, the beam falling upon the photoconductive layer ise'ltective in reducing the impedance of that portion of the layerthereby providing a larger percentage of the source voltage across theselected region of the electroluminescent layer.

It is also within the purview of the present invention to utilize a lensin place of the mask 32 as shown in Figure 3.

The lens may be of the plano-convex variety as illustrated or may be ofany suitable lenticular type adapted to provide an optical arrangementfor directing the illumination from a plurality of sources to impingeupon selected portions of photoconductive layer 12.

It is, of course, to be understood that the geometry of the entiresystem must be selected to provide the desired intersection of the beamsor illumination at only the appropriate areas of the device It That is,in the use of a mask, as illustrated in Figure 1, the source to maskdistance must be related to the mask to electroluminescent devicedistance in such a manner to effect illumination of only one of thecolor reproducing areas by one of the beams.

This is also true when utilizing an optical device or lens 41 as shownin Figure 3. The lens spacing from both the device it) and the source ofillumination must be such as to allow a concentration and an impingementof a particular beam upon a portion of the photoconductive layer 12which is adjacent to a strip of electroluminescent material adapted toproduce the particular color representative of the signal informationwith which the particular beam is modulated. It is also to be understoodthat the line type structure illustrated in Figures 1, 2 and 3 may be ofeither the horizontal line type or the vertical line type depending onthe particular system utilized.

In Figure 4, there is shown an arrangement wherein the source ofillumination such as the cathode-ray tubes 20, 21 and 22 are placed in atriangular arrangement at a particular distance from an aperture mask 42having therein a plurality of circular apertures adapted to allow thepassage of the beam from a particular one of the sources of illuminationwhich is directed toward a portion of the photoconductive layer 12 whichis adjacent to an elemental area of electroluminescent material selectedto provide light of the particular color desired. In Figure 4, only oneof the apertures is illustrated for the purpose of simplicity, however,in Figure 5 there is shown a cut-away portion of the aperture mask 42having therein a plurality of circular apertures positioned above acut-away portion of the electroluminescent layer 11 to illustrate theregistration of the mask 42 with the corresponding areas of theelectroluminescent device 10.

It may be seen from the examination of Figure 5 that the geometry of themask 42 and of the electroluminescent device 10 is such as to provideillumination of only one red, green or blue area of theelectroluminescent layer 11. With this type of construction, it isnecessary to provide a plurality of recurring subelemental color areasin two directions. This may be accomplished as illustrated by the use ofcircular dot areas repeating in color sequence in a predetermined orderas determined by the color system utilized. For example, the colorsystem illustrated utilizes three sources of illumination and threeprimary colors for the reproduction of a color image. It is thereforenecessary to provide triads of color dots regularly recurring in twodirections throughout the area of electroluminescent layer 11.

It may be further noted that it may be desirable to provide an aperturediameter which is somewhat less than the diameter of the correspondingsubelemcntal color areas. This will reduce the problem of preciseregistration of each of the plurality of of beams with the sub-elementalcolor areas. Accordingly, a small misregistration will still allow theentire beam to impinge upon a portion of the photoconductive layer 12which is associated with only the particular color desired.

Light images in natural color may accordingly be reproduced by elementalcontrolling of the electric held across a layer of electroluminescentmaterial in accordance with a plurality of electrical signals, byutilizing a plurality of individual beams to selectively control theimpedance of elemental regions of a photoconductive layer placed inclose association with an electroluminescent layer. Each of the beamsmay be individually and separately modulated with signals represenativeof the component-colors of a televised image and may be developed andcontrolled through synchronized scanning and the use of a foraminousmember or optical device to irradiate only those portions of thephotoconductive layer associated with the electroluminescent materialcapable of producing the given color.

Having thus described the present invention, what is claimed is:

1. Apparatus for reproducing light images in color in accordance with acolor signal containing information representative of a plurality ofcolor components derived by scanning asubject and synchronizinginformation pertaining to said scanning comprising, in combination: asource of said color signal; means for deriving from said color signalseparate component color signals respectively representative of each ofsaid plurality of color components, a plurality of sources ofillumination of the cathode-ray type for producing substantially pointsources of illumination, said sources of illumination having a fixedpredetermined spatial relationship with respect to each other; aplurality of signal-responsive means each respectively connected with adifferent one of said sources of illumination for changing the intensityof said illumination in response to an applied signal, means coupling adifferent one of said separate component color signals respectively to adifferent one of said signal-responsive means, scanning deflection meansconnected with each of said sources of illumination for causing each ofsaid pointsources of illumination to scan a raster, synchronizing meansreceiving said synchronizing information connected with said scanningdeflection means to synchronize the deflection of each of saidpoint-sources of illumination with the scanning of said subject; opticalmeans coupled to said sources of illumination for achieving opticalregistry of said rasters at the focal plane of said optical means, alight-sensitive color-image-producing target positioned in the opticalpath of said sources of illumination, said target comprising a layer ofprotoconductive material and a layer of electroluminescent material inclose operative relationship: a source of alternating current; meanscoupling said alternating-current source across said two layers inseries relation to produce an electric field across said target; saidtarget being positioned such that said photoconductive layer lies insaid focal plane whereby said photoconductive layer interceptsillumination from each of said point sources, said electroluminescentlayer comprising a plurality of light-emitting phosphor depositsarranged in separate geometrical patterns, the number of deposits ineach pattern corresponding to the number of said plurality of colorcomponents, each deposit in a given pattern being responsive to energyconditionally directed upon it from a given one of said illuminationsources to produce a different color of light emission correspondingrespectively to the color represented by a different one of said colorcomponent signals; and an illumination-source selection means interposedbetween said sources of illumination and said target to directillumination from the source of illumination associated with each ofsaid component color signals only onto the area of the photoconductivelayer adjacent to the phosphor deposits in each pattern having lightemission of the corresponding color.

2. Apparatus for reproducing light images in color in accordance with acolor signal containing information representative of a plurality ofcolor components derived by scanning a subject and synchronizinginformation pertaining to said scanning comprising, in combination: asource of said color signal; means for deriving from said color signalseparate component color signals respectively representative of each ofsaid plurality of color components, a plurality of sources ofillumination of the cathode-ray type for producing substantially pointsources of illumination, said sources of illumination having a fixedpredetermined spatial relationship with respect to each other; aplurality of signal-responsive means each respectively connected with adifierent one of said sources of illumination for changing the intensityof said illumination in response to an applied signal, means coupling adiiferent one of said separate component color signals respectively to adifferent one of said signal-responsive means, scanning deflection meansconnected with each of said sources of illumination for causing each ofsaid pointsources of illumination to scan a raster, synchronizing meansreceiving said synchronizing information connected with said scanningdeflection means to synchronize the deflection of each of saidpoint-sources of illumination with the scanning of said subject; opticalmeans coupled to said sources of illumination for achieving opticalregistry of said rasters at the focal plane of said optical means, alight-sensitive color-image-producing target positioned in the opticalpath of said sources of illumination, said target comprising a layer ofphoto-conductive material and a layer of electroluminescent material inclose operative relationship; a source of alternating current; meanscoupling said alternating-current source across said two layers inseries relation to produce an electric field across said target; saidtarget being positioned such that said photoconductive layer'lies insaid focal plane whereby said photoconductive layer interceptsillumination from each of said point sources, said electroluminescentlayer comprising a plurality of light-emitting phophor deposits arrangedin separate geometrical patterns, the number of deposits in each patterncorresponding to the number of said piurality oi color components, eachdeposit in a given pattern being responsive to energy conditionallydirected upon it from a given one of said illumination sources toproduce a different color of light emission corresponding respcctiveiyto the color represented by a difierent one of said color componentsignals; said phosphor deposits being in the form of strips, each ofsaid patterns comprising successive strips of difierent colorlight-emitting phosphor deposits in a predetermined order and arrangedin parallel relationship with respect to one another; and anillumination-source selection means interposed between said sources ofillumination and said target to direct illumination from the source ofillumination associated with each of said component color signals onlyonto the area of the photoconductive layer adjacent to the phosphordeposits in each pattern having light emission of the correspondingcolor, said selection means comprising a thin plate having aperturesspaced proportionally to the width of said patterns, said aperturesbeing shaped to restrict the light emanating from each separateillumination source to the areas of said photoconductive layerassociated with the separate corresponding color light-emitting phosphordeposit strips.

3. Apparatus for reproducing light-images in color in accordance with acolor signal containing information representative of a plurality ofcolor components derived by scanning a subject and synchronizinginformation pertaining to said scanning comprising, in combination: asource of said color signal; means for deriving from said color signalseparate component color signals respectively representative of each ofsaid plurality of color components, a plurality of sources ofillumination of the cathode-ray type for producing substantiallypointsources of illumination, said sources of illumination having afixed predetermined spatial relationship with respect to each other; aplurality of signal-responsive means each respectively connected with adifferent one of said sources of illumination for changing the intensityof said illumination in response to an applied signal, means coupling adifierent one of said separate component color signals respectively to adifferent one of said signal-responsive means, scanning deflection meansconnected with each of said sources of illumination for causing each ofsaid point-sources of illumination to scan a raster, synchronizing meansreceiving said synchronizing information connected with said scanningdeflection means to synchronize the deflection of each of saidpointsources of illumination with the scanning of said subject; opticalmeans coupled to said sources of illumination for achieving opticalregistry of said rasters at the focal plane of said optical means, alight-sensitive color-image-producing target positioned in the opticalpath of said sources of illumination, said target comprising a layer ofphotoconductive material and a layer of electroluminescent material inclose operative relationship; a source of alternating current; meanscoupling said alternating-current source across said two layers inseries relation to produce an electric field across said target; saidtarget being positioned such that said photoconductive layer lies insaid focal plane whereby said photoconductive layer interceptsillumination from each of said point sources, said electroluminescentlayer comprising a plurality of light-emitting phosphor depositsarranged in separate geometrical patterns, the number of deposits ineach pattern corresponding to the number of said plurality of colorcomponents, each deposit in a given pattern being responsive to energyconditionally directed upon it from a given one of said illuminationsources to produce a diiferent color of light emission correspondingrespectively to the color represented by a different one of said colorcomponent signals; said phosphor deposits being in the form of strips,each of said patterns comprising successive strips of different colorlight-emitting phosphor deposits in a predetermined order and arraged inparallel relationship with respect to one another; and anillumination-source selection means interposed between said sources ofillumination and said target to direct illumination from the source ofillumination associated with each of said component color signals onlyonto the area of the photoconductive layer adjacent to the phosphordeposits in each pattern having light emission of the correspondingcolor, said selection means comprising a plurality of lenses interposedin said optical path, said lenses being of the type having a cylindricalsurface and having a width proportional to said patternwidth and alength substantially equal to the length of said strips, saidcylindrical surface having the proper curvature to produce the correctrefraction of the illumination from said illumination sources to directthe illumination from each separate illumination source ontocorresponding color light-emitting phosphor deposit strips.

4. Apparatus for reproducing light images in color in accordance with acolor signal containing information representative of a plurality ofcolor components derived by scanning a subject and synchronizinginformation pertaining to said scanning comprising, in combination: asource of said color signal; means for deriving from said color signalseparate component color signals respectively representative of each ofsaid plurality of color components, a plurality of sources ofillumination of the cathode-ray type for producing substantially pointsources of illumination, said sources of illumination having a fixedpredetermined spatial relationship with respect to each other; aplurality of signal-responsive means each respectively connected with adifferent one of said sources of illumination for changing the intensityof said illumination in response to an applied signal, means coupling adifferent one of said separate component color signals respectively to adilierent one of said signal-responsive means, scanning deflection meansconnected with each of said sources of illumination for causing each ofsaid point-sources of illumination to scan a raster, synchronizing meansreceiving said synchronizing information connected with said scanningdeflection means to synchronize the deflection of each of saidpoint-sources of illumination with the scanning of said subject; opticalmeans coupled to said sources of illumination for achieving opticalregistry of said rasters at the focal plane of said optical means, alight-sensitive color-image-producing target positioned in the opticalpath of said sources of illumination, said target comprising a layer ofphotoconductive material and a layer of electroluminescent material inclose operative relationship; a source of al- 10 ternating current;means coupling said alternating-current source across said two layers inseries relation to produce an electric field across said target; saidtarget being positioned such that said photoconductive layer lies insaid focal plane whereby said photoconductive layer interceptsillumination from each of said point sources, said electroluminescentlayer comprising a plurality of light-emitting phosphor dot depositsarranged in separate geometrical patterns, the number of deposits ineach pattern corresponding to the number of said plurality of colorcornponents, each deposit in a given pattern being responsive to energyconditionally directed upon it from a given one of said illuminationsources to produce a different color of light emission correspondingrespectively to the color represented by a different one of said colorcomponent signals, each of said geometrical patterns being an elementalarea of said electroluminescent layer and each phosphor deposit being ofa size substantially of the same order as the point of light projectedfrom said source of illumination as it strikes the photoconductivelayer, said dot deposits arranged Within said patterns in a colorsequence of a predetermined order; .and an illuminationsource selectionmeans interposed between said sources of illumination and said target todirect illumination from the source of illumination associated with eachof said component color signals only onto the area of thephotoconductive layer adjacent to the phosphor deposits in each patternhaving light emission of the corresponding color.

References Cited in the file of this patent UNITED STATES PATENTS2,072,455 Kannenberg Mar. 2, 1937 2,605,335 Greenwood July 29, 19522,650,310 White Aug. 25, 1953 2,710,890 Skellett June 14, 1955 2,728,815Kalfaian Dec. 27, 1955 OTHER REFERENCES Journal of the Optical Societyof America, vol. 44, No. 4, pages 297-299; April 1954.

